This invention relates to greases and, more particularly, to lithium soap thickened greases.
In lubrication of engine bearings of cruise missiles, low temperature requirements are severe, requiring good performance during start-up at temperatures as low as -65.degree. F. Other demands include improved rust protection even when exposed to humid conditions for years. Also, oil separation from the bearings must be minimized for extended periods of time since cruise missiles may be stored for years before being used. In addition, outstanding oxidation stability is required to assure maintenance of the grease's bearing lubrication properties. Furthermore, after very long periods of storage under humid conditions and/or low temperature, the grease must provide excellent bearing lubrication during sudden start-up and continue to provide such protection during the entire flight of the missile. Failure to perform in this manner could cause the cruise missile to fail to reach its objective and instead terminate its flight at an unintended and unknown site. Clearly, such an occurrence could be disastrous.
Actual performance demands of the lubricating grease during extended storage, start-up, and flight of the cruise missile are quite extensive and cannot be measured by any of the typical bench tests commonly used to evaluate lubricating greases. Special test rigs designed specifically to simulate conditions experienced by the bearing during storage, firing, and flight of strategic cruise missiles are required to insure that the grease will perform adequately.
Conventional mineral oil-based greases are limited in their usefulness in low temperature applications. For example, greases made from paraffinic mineral oil often provide below average performance at low temperatures because of wax which is usually present in the grease. At temperatures below 0.degree. F., wax can crystallize out and render the grease hard and non-pliable. Dewaxing processes can reduce the wax level in paraffinic mineral oil but cannot eliminate it altogether. Naphthenic mineral oils have virtually no wax and have better low temperature flow properties, but do not give good flow properties at extremely low temperatures, such as -40.degree. F. to -65.degree. F. Also, naphthenic oils are more prone to oxidative and thermal degradation at high temperatures.
In lithium soap thickened greases, the metal base, usually lithium hydroxide or in its more commonly available form of lithium hydroxide monohydrate, is reacted with a fatty acid, usually 12-hydroxystearic acid, or with a fatty acid derivative, usually methyl 12-hydroxystearate or hydrogenated castor oil. This reaction is most often carried out in the base oil with water also being present. The water is added to act as a reaction solvent if the acid is used. If the fatty acid derivative is used, the water acts both as reaction solvent and reactant, the latter effect being necessary for the hydrolytic cleavage of the ester linkages in the methyl 12-hydroxystearate or the hydrogenated castor oil.
Typifying some of the many types of prior art lubricating oils, greases, and additives, are those described in U.S. Pat. Nos. 3,622,512; 3,853,775; 3,876,550; 3,890,363; 4,514,312; 4,536,308; 4,735,146; 4,759,859; 4,787,992; 4,830,767; 4,858,534; 4,859,352; 4,879,054; 4,902,435; and 4,904,399. These prior art lubricating oils, greases, and additives have met with varying degrees of success but have not been demonstrated to perform well for cruise missile engine bearings.
It is, therefore, desirable to provide an improved lithium grease for use in cruise missile bearings.